Cadmium-induced expression of metallothionein and suppression of RNA to DNA ratios during molluscan development

Effects of cadmium on cellular protein and glutathione synthesis and expression of stress proteins in eastern oysters, Crassostrea virginica Gmelin

Cadmium (Cd) is an important toxicant in estuarine and coastal environments that can strongly affect energy balance of aquatic organisms by increasing the organism's basal energy demand and reducing its aerobic capacity. Mechanisms of cadmium-induced increase in basal metabolic costs are not well understood and may involve elevated detoxification costs due to the synthesis of cellular protective proteins and glutathione. We studied the short-term effects of cadmium exposure (4 h) on protein and glutathione (GSH) synthesis and expression of stress proteins (heat shock proteins HSP60, HSP70 and HSP90) and metallothioneins in isolated gill and hepatopancreas cells of the eastern oyster, Crassostrea virginica. Our study showed that exposure to cadmium resulted in a dose-dependent increase in the rate of protein synthesis in oyster cells, which reached 150% of the control at the highest tested Cd level (2000 micromol l(-1)). GSH synthesis was significantly inhibited by the highest Cd concentrations, especially in hepatopancreas, which resulted in a slight but significant decrease in the total GSH concentrations. Elevated protein synthesis was associated with the increased expression of metallothioneins and heat shock proteins. Interestingly, stress protein response differed considerably between gill and hepatopancreas cells. In hepatopancreas, expression of metallothionein mRNA (measured by real-time PCR) increased 2-8-fold in response to Cd exposure, whereas no significant increase in metallothionein expression was found in Cd-exposed gill cells. By contrast, HSP60 and HSP70 protein levels increased significantly in Cd-exposed gill cells (by 1.5-2-fold) but not in hepatopancreas. No change in HSP90 expression was detected in response to Cd exposure in oyster cells. These data indicate that metallothionein expression may provide sufficient protection against Cd-induced damage to intracellular proteins in hepatopancreas, alleviating the need for overexpression of molecular chaperones. By contrast, Cd detoxification mechanisms such as inducible metallothioneins and GSH appear to be insufficient to fully prevent protein damage in gill cells, thus necessitating induction of HSPs as a secondary line of cellular defense. Therefore, gills are likely to be among the most Cd-sensitive tissues in oysters, which may have important implications for impaired oxygen uptake contributing to energy misbalance and reduced aerobic scope in Cd-exposed oysters.

Metal bioaccumulation and metallothionein concentrations in larvae of Crassostrea gigas

Larval stages of bivalve molluscs are highly sensitive to pollutants. Oysters from a hatchery from Normandy (English Channel) were induced to spawn, and fertilized eggs were exposed to copper or cadmium for 24 h. Metal accumulation (from 0.125 to 5 microg Cu L(-1) and from 25 to 200 microg Cd L(-1)) and MT concentrations were measured in larvae. Compared to controls, larvae accumulated copper and cadmium with an increase in MT concentrations particularly with cadmium (i.e. 130.96 ng Cu (mg protein)(-1) and 12.69 microg MT (mg protein)(-1) at 1 microg Cu L(-1) versus 23.19 ng Cu (mg protein)(-1) and 8.92 microg MT (mg protein)(-1) in control larvae; 334.3 ng Cd (mg protein)(-1) and 11.70 microg MT (mg protein)(-1) at 200 microg Cd L(-1) versus 0.87 ng Cd (mg protein)(-1) and 4.60 microg MT (mg protein)(-1) in control larvae). Larvae were also obtained from oysters of a clean area (Arcachon Bay) and a polluted zone (Bidassoa estuary) and exposed to copper in the laboratory, their MT concentration was measured as well as biomarkers of oxidative stress. Biomarker responses and sensitivity to copper for the larvae from Arcachon oysters were higher than for those from Bidassoa.

Quantitative reverse transcription polymerase chain reaction of a molluscan metallothionein mRNA

A quantitative assay based on competitive reverse transcription polymerase chain reaction (RT-PCR) was developed for metallothionein (MT) mRNA of the mollusc Crassostrea virginica and applied to analysis of MT mRNA of hemocytes. The assay was based on titration of a competitive external standard cRNA derived from the coding region of the oyster MT mRNA. Serial dilutions of the cRNA standard were coamplified with a constant amount of total RNA using biotinylated primers common to both target and standard sequences. Amplified products were bound to streptavidin-coated plates and hybridized to sequence-specific fluorescein-labeled probes. Detection was based on single photon counting of chemiluminescence generated by an alkaline phosphatase-conjugated antifluorescein antibody. For quantification, the target chemiluminescence was normalized to that of the standard, and the amount of target MT mRNA in the sample was derived from the titration. Cadmium-induced MT mRNA equivalent to that in 180 hemocytes was easily detected, and, for routine quantitative analysis, was sufficiently sensitive to quantify basal and induced MT mRNA. Basal hemocyte MT mRNA of 133+/-8 (1 S.E.) amol per microgram total RNA was induced 5-fold to 573+/-14 amol per microgram total RNA by in vitro exposure to 15 microM CdCl(2) for 20 h.

Effects of glutathione depletion on copper cytotoxicity in oysters (Crassostrea virginica)

Glutathione is a tripeptide that plays an important role in ameliorating metal toxicity. Depletion of glutathione has been associated with an increased risk of metal toxicity in mammals. An understanding of the toxicological significance of glutathione depletion in oysters would be of considerable importance given the widespread use of bivalves in biological monitoring. Laboratory studies were conducted by using an inhibitor of glutathione synthesis (buthionine sulfoximine) to investigate the effects of glutathione depletion on metallothionein expression, lysosomal membrane destabilization, and lipid peroxidation in Cu-exposed oysters. In oysters exposed to Cu (20 and 80 µg/l) and buthionine sulfoximine (20 mg/l), metallothionein induction was suppressed and cellular stress responses were frequently higher than those observed in oysters exposed singly to Cu. Together, these results suggest that environmental conditions that cause glutathione depletion may increase the potential for adverse effects to pollutants during in situ exposures.

Metallothionein mRNA induction and generation of reactive oxygen species in molluscan hemocytes exposed to cadmium in vitro

(1) Hemocytes of the oyster Crassostrea virginica were exposed to Cd in vitro to examine the concomitant induction of metallothionein (MT) mRNA and production of reactive oxygen species (ROS) during the oxidative burst response of phagocytic cells. (2) MT mRNA induction was concentration-dependent, exhibiting a threshold between 5 and 10 microM cdCl2, and an increase at higher Cd concentrations up to a maximum level of 36 microM cdCl2. Continued increases in the Cd exposure concentrations resulted in declines in the levels of MT mRna. (3) No significant effects of Cd on ROS production were observed at Cd concentrations up to 36 microM CdCl2. However, ROS production decreased in a concentration-dependent fashion at CdCl2 concentrations from 36 to 356 microM, the highest concentration tested. (4) At these higher concentrations of Cd, the concomitant decrease in MT mRNA inducibility and suppression of ROS production were probably manifestations of the general cytotoxicity of Cd. ROS suppression can result in reduced resistance to infectious agents, suggesting that Cd is immunotoxic to C. virginica.